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Chapter 4. Accessory Hacks > Enhance Nebular Contrast and Detail

Hack 59. Enhance Nebular Contrast and Detail

Make the faint fuzzies a bit fainter but a lot less fuzzy.

As useful as color filters can be for Lunar and planetary observing, they're no help at all for observing nebulae. Nebulae are so dim that only the very brightest of them show even a hint of color, so the only thing a standard color filter does is dim them further.

Wouldn't it be nice if there were filters that selectively passed the light emitted by nebulae while stopping light pollution and other "bad" light? As it happens, such nebula filters do exist. Like any filter, nebula filters can't add anything; they can only take away. But, by selectively removing undesirable light while passing nearly all of the light emitted by nebulae, nebula filters enhance the level of contrast and detail visible in nebulae, even from a dark-sky site.

For an in-depth discussion of the technical aspects of nebula filters, read Choosing a Nebula Filter by Greg A. Perry, Ph.D. (http://members.cox.net/greg-perry/filters.html).


4.17.1. Dimming Background Skyglow

Broadband filters, often called Light Pollution Reduction filters or LPR filters, are one of a class known as interference filters. Unlike color filters, which are monolithic slabs of dyed glass, interference filters use multiple microscopically thin interference coatings to selectively block or pass specific wavelengths. Depending on the number and type of interference coatings applied, the passbands of the filter may be broad or narrow, and contiguous or separated. Figure 4-32, for example, shows the transmission curve of the Orion SkyGlow, a typical broadband filter. Similar broadband filters are available under various tradenames from Parks/Lumicon, Thousand Oaks, and other astronomy vendors.

Figure 4-32. Transmission curve of the Orion Skyglow broadband filter


While stars have continuous emission spectra, various astronomical objects, such as planetary nebulae and emission nebulae, emit light only at certain wavelengths, which include:

  • The bluish hydrogen-beta (H-beta or H-β) line at 486 nm

  • The blue-greenish doubly ionized oxygen (Oxygen-III or O-III) lines at 496 and 501 nm

  • The blue-greenish cyanogen (CN) lines at 511 and 514 nm

  • The red hydrogen-alpha (H-alpha or H-α) line at 656 nm

Broadband filters have relatively wide passbands in the blue/blue-green and red parts of the spectrum to allow these astronomically important emission lines to pass with little or no attenuation.

Conversely, broadband filters are designed to block portions of the spectrum lower than about 445 nm and between about 540 nm and 640 nm because those portions of the spectrum include the emission lines of major sources of light pollution, including:

  • The purplish mercury (Hg) lines at 405 and 436 nm produced by mercury-vapor lights

  • The yellow/yellow-orange mercury (Hg) lines at 546 and 579 nm produced by mercury-vapor lights

  • The yellow line at 558 nm produced by natural air glow

  • The yellow/orange sodium (Na) lines at 570, 579, 583, and 617 nm produced by low-pressure sodium-vapor lights

It is these "bad" emission lines that cause the pinkish-yellow skyglow in heavily light-polluted locations. In theory, then, a broadband filter should stop all of the "bad" light and pass all of the "good" light, allowing you to observe under even relatively bright suburban skies. And, in fact, broadband filters are usually marketed as "light pollution reduction" filters, suitable for use under moderately light-polluted conditions. Alas, the reality is different.

The real problem is that line-spectrum sources of light pollution, such as mercury-vapor and low-pressure sodium-vapor lights, are rapidly being replaced by continuous-spectrum lighting, primarily high-pressure sodiumvapor lights. This is occurring for both esthetic and economic reasons.

Esthetically, most people dislike the stark yellow brilliance of low-pressure sodium-vapor lights and the purple glow of mercury-vapor lights. (Also, mercury-vapor lights use the environmentally unsafe mercury, which presents disposal problems.) Economically, high-pressure sodium-vapor lights are more efficient than the older low-pressure sodium-vapor and mercury-vapor lights, which means they use less electricity and cost less to run for a given light output.

As light pollution sources increasingly shift to continuous-spectrum output, the value of broadband filters continues to shrink. Although they may provide minor image enhancement under bright urban and suburban skies by blocking most of the wavelengths emitted by sodium and mercury lights, few people consider the marginal improvement worth the relatively high cost of these filters, typically $60 to $100 in 1.25" versions and as much as $200 in 2" versions.

A broadband filter is most helpful with objects such as emission nebulae and planetary nebulae that emit most or all of their light at specific wavelengths. This is true because the broadband filter selectively dims unwanted wavelengths while allowing desirable wavelengths to pass with little or no diminution, thereby increasing the contrast of the object against the sky.

Continuous emitters, objects that emit across the entire visible spectrum— including stars, galaxies, and reflection nebulae—show less improvement with a broadband filter because the filter also blocks part of the light they emit. Still, because a broadband filter blocks "bad" wavelengths selectively, it does enhance contrast for galaxies and reflection nebulae, although the improvement is usually quite subtle.

Under typical urban observing conditions, light pollution is so severe that it effectively "swamps" the ability of a broadband filter to selectively block undesirable wavelengths. Although the broadband filter still eliminates sodium and mercury wavelengths, the sky remains so brightly lit by continuous-spectrum sources that it is difficult to locate faint fuzzies, let alone see them. The filtered view is marginally superior to the unfiltered view, but the improvement is so subtle that most observers won't consider a broadband filter very helpful for urban observing.

A broadband filter is more useful at darker site. Because it selectively blocks natural skyglow and the wavelengths emitted by sodium and mercury lights, a broadband filter can darken the sky background, increasing contrast and providing a more esthetically pleasing view. Although the effect of a broadband filter is subtle, it can improve contrast and reveal additional detail in continuous emitters like galaxies and reflection nebulae from darker sites.

A broadband filter is no substitute for a dark observing site. If you buy a broadband filter expecting it to magically slay light pollution and allow you to observe dim galaxies from beneath a streetlight, you'll be disappointed. But if you accept what it offers—selective blocking of most undesirable wavelengths—and use it appropriately, you'll find that a broadband filter provides marginal improvement for many objects.

There is one activity for which a broadband filter is nearly indispensable. For film imaging at a site with slight light pollution, using a broadband filter extends the exposure time you can use while maintaining a dark-sky background, thereby allowing your images to reveal dimmer objects and more detail in brighter objects.


4.17.2. Enhancing Nebular Contrast

Narrowband filters, such as the Orion UltraBlock and the Parks/Lumicon Ultra-High Contrast (UHC), use the same interference-layer coating technology as broadband filters. They differ, as you might expect from their name, in the width and location of their passbands. Figure 4-33 shows the passband for an Orion UltraBlock narrowband filter. (Compare this figure to the preceding broadband transmission curve.)

Figure 4-33. Transmission curve of the Orion UltraBlock broadband filter


Narrowband filters have very high transmission at the critical wavelengths of 486 nm (H-β) and 496/501 nm (O-III) and almost zero transmission outside that narrow band of spectrum. (Some narrowband filters also pass the 656 nm H-αline, but that wavelength seldom contributes much to the view unless you are using a large scope.) Because planetary nebulae and most emission nebulae emit most or all of their light in the passband of narrowband filters, viewing these objects using a narrowband filter selectively increases their contrast against the background sky, often dramatically.

With a narrowband filter, the background sky is darkened dramatically. You can see more of the extent of the object because dimmer portions of it that would otherwise be obscured by natural or artificial skyglow are visible with the filter blocking those parts of the spectrum. More fine, low-contrast detail is visible because you are looking only at the light emitte by the object, unsullied by light from other sources. In addition to improving the view of many objects, a narrowband filter also reveals objects that are entirely invisible without it. For example, from a reasonably dark site, we can glimpse the Rosette Nebula in Monoceros naked eye by looking through a narrowband filter.

The degree of improvement provided by a narrowband filter is strongly influenced by observing conditions. If you are not fully dark adapted [Hack #11], a narrowband filter may show no improvement at all because much of the enhancement occurs in the fainter portions of the nebulosity.

Exit pupil size also matters. Narrowband filters provide the most enhancement with an exit pupil in the range from 7mm down to 2mm, which translates to 3.5X to 12.5X magnification per inch of aperture. Best results are in the 3.5X to 8X per inch range.


Because a narrowband filters have such narrow passbands, they significantly darken not just skyglow but any continuous-spectrum light source, including stars, galaxies, and reflection nebulae (which glow by reflected starlight). Accordingly, they truly are "light pollution reduction" filters, but the cure may kill the patient.

That is, although a narrowband filter can eliminate or greatly attenuate light pollution even under bright urban skies, it does so by dimming the view dramatically. So much so that it may be difficult to see anything except line-emitting objects like emission nebulae, including the very stars that you need to see to guide you to the object. With anarrowband filter, bright stars appear dim, and dim stars are invisible.

Because a narrowband filter makes it difficult to see any but the brightest stars, the proper technique is to get the object in the field of view first, and then use the filter to view the object, either by screwing thefilter onto the eyepiece or by "blinking" the object, which is to say holding the filter between the eyepiece and your eye and moving it in and out of your line of view.


Regardless, the benefits of a narrowband filter are indisputable. At light-polluted sites, a narrowband filter allows you to view planetaries and emission nebulae against a pleasingly dark sky background. At dark sites, the narrowband provides images significantly better than unfiltered views. We think a narrowband filter belongs in every DSO observer's eyepiece case.

We suggest you not dither about which brand to buy. The two market leaders are the Orion UltraBlock and the Parks/Lumicon UHC. Both are excellent products, and their similarities greatly outweigh their differences. We have done detailed A–B comparisons of the two, attaching them to two identical 14mm Pentax XL eyepieces and then swapping the eyepieces in and out of the same 10" scope. Our colleagues agree with us: although there are minor differences in the views, they're not worth worrying about. Buy whichever happens to be cheaper or on sale at the time. You'll be happy with either.

One night, Robert and our friend Paul Jones decided to do an A–B test of the UltraBlock against the UHC. Robert mounted his UltraBlock on his 14mm Pentax XL eyepiece.Paul mounted his UHC on his 14mm Pentax XL. We pointed Robert's 10" Dob at M42, the Great Orion Nebula, and began swapping the eyepieces in and out. Robert slightly preferred the view in the UltraBlock and Paul slightly preferred the view with the UHC—probably pride of ownership—but both agreed that the differences were almost unnoticeable. Barbara took a quick look at each and announced she much preferred our UltraBlock.

So Paul decided to call her bluff. He used his body to conceal which eyepiece/filter he was putting into the focuser. He then called Barbara over and asked her which filter was in place. "That's ours!", Barbara announced emphatically. Paul and Robert just looked at each other, thinking "how could she possibly tell?" So we asked her. Barbara informed us that the fourth star in the Trapezium was easily visible with our UltraBlock, but very faint with Paul's UHC. We checked, and sure enough it was true. Barbara notices things.


4.17.3. Blocking All But a Specified Wavelength

Line filters can be thought of as narrowband filters on steroids. While a typical broadband filter has a 100 nm passband, and a typical narrowband filter has a 25 nm passband, a line filter may have a passband of only 8 nm or so. Line filters isolate one particular line (or a closely grouped pair of lines), transmitting nearly all of the light at that wavelength and blocking all other light. Accordingly, the view through a line filter is quite dim.

There are two line filters in common use among amateur astronomers:


Oxygen-III

The Oxygen-III filter, also called an O-III filter or O-3 filter, is useful for many emission nebulae, but its most common use is to enhance the view of planetary nebulae, many of which emit light almost exclusively on the 496 nm and 501 nm lines of doubly ionized oxygen. Although a standard narrowband filter enhances many planetary nebulae, the O-III filter does a much better job on some planetaries, sometimes dramatically so. An O-III filter is usually the second choice of dedicated DSO observers, after the indispensable narrowband filter.

There are no hard-and-fast rules as to which is the best filter for observing a particular object or class of objects. Some planetaries, for example, reveal more detail with a narrow-band filter than with an O-III filter. Conversely, some emission nebulae are shown to better advantage with an O-III filter than with a narrowband filter.



Hydrogen-beta

The Hydrogen-beta filter, also called an H-beta filter, H-βfilter,or Hydrogen-b filter, is often referred to as the Horsehead Nebula filter, only partly in jest. The H-beta filter is extremely specialized. It is useful for only a handful of objects, of which the Horsehead Nebula is by far the most famous. But for those few objects, the H-beta filter is nearly essential. It's not overstating the case much to say that without an H-beta filter the Horsehead is elusive visually even in large scopes, while with an H-Beta filter the Horsehead becomes possible—although challenging—from a dark site with scopes as small as 8" to 12", and relatively easy in large scopes.

We consider the O-III filter the second most useful nebular filter overall, and one that any serious DSO observer will want to own. The H-beta filter is a different matter. If you are on a quest to view the Horsehead Nebula, by all means buy an H-beta filter of your own. Otherwise, there are better things to do with your money. Just borrow an H-beta filter from time to time when you need it.

Some interference filters, including those sold by Orion and Lumicon, sandwich their interference coatings between two layers of glass. These filters are as durable as standard dyed-glass color filters and can be cleaned safely just as you'd clean any other optical surface. Other interference filters, including those sold by Sirius and DGM optics, put their fragile coatings on an outside surface. These filters are extremely delicate and can be ruined even by careful cleaning. In fact, we have heard reports of such filters being damaged by dew.


4.17.4. Choosing Nebula Filters

There's no question that serious DSO observers should own and use nebula filters, but deciding how many you need, which types, and which brands is not easy. Fortunately, unless you are restricted by your budget, it's not an either-or situation. Buy more than one nebula filter, and use whichever provides the best view of the object you happen to be observing. We suggest the following guidelines:

  • If you're going to buy only one nebula filter, choose a narrowband filter such as the Lumicon UHC or the Orion UltraBlock. A narrowband filter is the most generally useful of the bunch. It is the best filter for most objects that benefit from using a nebular filter, and it shows a greater improvement on more objects than any other type of nebula filter. A narrowband filter provides at least some improvement on nearly all of the objects that benefit from any type of nebular filter. In short, a narrowband filter is most likely to help and least likely to hurt the view of any nebular object. If you observe DSOs, you should make it a high priority to acquire a narrowband filter.

  • If you buy a second nebula filter, choose an O-III filter. An O-III filter is the best choice for most planetary nebulae, and it improves many emission nebulae as well.

    Although most emission nebulae are more enhanced by the narrowband filter than the O-III filter, there are reflection nebulae that appear better with the O-III. Similarly, although the O-III is the best choice for most planetary nebulae, some planetaries are better served by the narrowband filter.

    To some extent, "best" is a matter of opinion. For example, when we tested the UltraBlock and UHC filters on M42 against the OIII, we were hard-pressed to say which provided the "best" view. They were different views, certainly. The extent of the nebula was larger with the narrowband filters, and the image was brighter, as you might expect. On the other hand, the OIII filter revealed fine detail that was not visible in either narrowband filter.

    If you can afford only one nebular filter, the narrowband is unquestionably the better choice (unless you happen to specialize in planetary nebulae). Otherwise, buy one of each.


  • Most DSO observers are content with just a narrowband filter, or with narrowband and O-III filters. But if there's room left in the budget for one more filter, add a broadband filter such as the Lumicon Deep-Sky or the Orion SkyGlow. There are some objects for which a broadband filter improves the view more than a narrowband or O-III filter. If the cost doesn't matter, buy a broadband filter and try it on various objects. But we suggest you consider the broadband filter a low-priority purchase.

  • Finally, there's the H-beta filter. For the California Nebula, the Cocoon Nebula, the Horsehead Nebula, and a dozen or so other objects, the H-beta filter is almost a magic bullet. But the H-beta filter is of very limited help on most emission nebulae, and actually degrades the view for most planetary nebulae. The H-beta is a very specialized filter, and we suggest that you consider acquiring one a very low priority.

4.17.5. Specialty Filters

Various specialty filters exist that have even more narrowly defined uses than the H-Beta filter. For example, the Parks/Lumicon Swan-Band Comet filter is for observing comets, period. Its passband includes the cyanogen lines and the 501 nm O-III line, but not the 496 nm O-III line. This filter is useless unless you observe comets; if you do, it's very useful indeed.

Dedicated amateur astronomer Al Misiuk also happens to be an expert on interference coatings. He formed a company named Sirius Optics (http://www.siriusoptics.com), which produces various specialized interference filters. The first project Al took on was making a filter to improve the false color generated by achromatic refractor when viewing bright objects. Traditionally, astronomers used a yellow filter to reduce this blue-violet haze, but the transmission characteristics of Wratten color filters are not ideal for this purpose. The Sirius Optics MV-1 (minus violet) interference filter eliminates most false color while imparting only a mild greenish-yellow cast. The enhanced Neodymium Eyepiece Filter combines MV-1 interference coatings with neodymium optical glass to provide an almost completely neutral image. The MARS 2003 filter is an interference filter optimized for (you guessed it) Mars, although it is also useful on some other objects. The NIR1 Near Infrared Blocking filter is useful for astrophotographers.

In addition to specialty filters, Sirius Optics also produces more general-purpose interference filters, including the CE1 Contrast Enhancement filter (enhanced broadband), the NEB1 Nebula Eyepiece Filter (narrowband), and the PC1 Planetary Contrast Eyepiece Filter.


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